I think you should have the replaceable resistor ends. They should be 5K ohms and the wires themselves maybe 1K???. The resistor ends unscrew from the plug wire, carefully roll the boot back and unscrew the resistor end, then check the ends and wires again. Be sure to wiggle the wires while testing for intermittent fluctuations. That is providing you still have original style wires/ends.
BTW: Nice mileage figures. I'm happy to touch on 16MPG @75-80 in the 4.5.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.